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I presume you have been offered freelance work by an organisation other than your employer. Please confirm this is the case and i will respond accordingly.
Thank you for your reply.
First of all, you need to register for self assessment to enable you to file a tax return unless you are already registered and file your tax return. For freelance work you are deemed self employed.
Registering is a relatively straight forward process and more information on this can be found here
As far as deduction of tax on freelance income goes.. if you are self-employed then you would receive your fee income gross in most cases (unless you are in construction industry). You will file your accounts on supplementary pages SA103 along with the main tax return on an annual basis. Your tax caluculation will be done once the tax return is submitted and you pay your tax on freelance income by 31 Jan of the following year. As an example say you file your tax return for current tax year sometime between Apr-Oct 2017. The tax on freelance income would be due by 31 Jan 2018. You would simply not deduct 20% of the income and send it to the tax man.
More information on how tax payments etc work in self assessment are covered here
I hope this is helpful and answers your question.
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The tax you pay on your full time employment should not alter.
Your total earnings are likely to be around £46,500. There would be some business expenses you could charge against your freelance income and let say the profit from freelance income is £3,250 after expenses.
This makes your income (42,500+3,250) £45,750. Your taxable income becomes (45,750-11,000) £34,750. About £2,750 would suffer tax at 40% and not 20% (this additional tax arising from income from freelance work).
I hope this is helpful.